The present invention relates to a hydrodynamic bearing unit. This bearing unit is preferably used particularly for supporting a spindle motor in information apparatus, for example, in a magnetic disk device such as an HDD, an FDD or the like, an optical disk device such as a CD-ROM, a DVD-ROM or the like, and a magneto-optical disk device such as an MD and an MO or the like, or for supporting a spindle in a polygon scanner such as a laser beam printer (LBP) or the like.
In the spindle motor of each kind of the above information apparatus, a higher speed, a lower cost, and a lower noise are demanded in addition to the high rotation precision or the like. One of the key elements that determine these performance requirements is a bearing for supporting a spindle of the motor. In recent years, as this kind of bearing unit, the usage of the hydrodynamic bearing with excellent characteristics is investigated to meet the demanded performance, or the bearing is actually used.
In spindle motors for the above information apparatus in recent years, the high rotation precision is more strongly demanded in an attempt to increase the information recording density and obtain higher speed rotation. In order to meet this demand, still higher rotation precision is demanded with respect to the hydrodynamic bearing incorporated in the above spindle motors.
As a factor which largely affects rotation precision of hydrodynamic bearings, clearance control in a radial bearing clearance and a thrust bearing clearance for generating dynamic pressure is considered important.
The present invention aims at providing a hydrodynamic bearing unit with a high rotation precision wherein clearance control of the radial bearing clearance and the thrust bearing clearance is made appropriate.
Furthermore, an object of the present invention is to suppress the friction of a radial bearing surface and a thrust bearing surface to maintain an excellent bearing performance in the hydrodynamic bearing unit described above.
The present invention provides a hydrodynamic bearing unit comprising: a shaft member having a shaft portion and a flange portion; a bearing member arranged on an outer circumference of the shaft member; and a radial bearing portion and a thrust bearing portion respectively comprising bearing surfaces with grooves (dynamic pressure grooves) for generating hydrodynamic pressure and bearing clearances facing the bearing surfaces, and supporting the shaft member in non-contact state, in a radial direction and a thrust direction respectively, with hydrodynamic pressure generated in the bearing clearances when the shaft member and the bearing member relatively rotate, wherein the squareness of the both end surfaces of the flange portion with respect to the outer circumference of the shaft portion of the shaft member is 0.001 mm or less respectively, and the flatness of the both end surfaces of the flange portion is 0.001 mm or less respectively.
The xe2x80x9cbearing memberxe2x80x9d in this invention includes a structure in which the bearing member having a radial bearing surface and a thrust bearing surface is fixed to a housing and a structure in which a radial bearing surface and a thrust bearing surface are directly formed on a housing.
In this case, it is desirable that the squareness of the end surface of the bearing member located opposite to one of the both end surfaces of the flange portion via the thrust bearing clearance with respect to the inner circumference of the bearing member is set to 0.002 mm or less while the flatness thereof is set to 0.0015 mm or less. Furthermore, the flatness of the surface located opposite to the other end surface of the flange portion via the thrust bearing clearance is set to 0.002 mm or less.
Furthermore, according to the present invention, in a hydrodynamic bearing unit comprising: a housing with a bottom portion; a bearing member fixed to an inner circumference of the housing; a shaft member having a shaft portion inserted into the inner circumference of the bearing member and a flange portion; a radial bearing portion provided between the inner circumference of the bearing member and the outer circumference of the shaft portion of the shaft member for supporting the shaft member in a radial direction in non-contact state with a dynamic pressure generated in a radial bearing clearance; and a thrust bearing portion respectively provided between each end surface of the flange portion of the shaft member and an end surface of the bearing member or an inner surface of the bottom portion of the housing for supporting the shaft member in a thrust direction in non-contact state with a dynamic pressure generated in the thrust bearing clearances, the flatness of the inner surface and the outer surface of the housing is 0.005 mm or less.
In addition to the above structure, the squareness of the both end surfaces of the flange portion with respect to the outer circumference of the shaft portion of the shaft member can be set to 0.001 mm or less while the flatness of the both end surfaces of the flange portion can be set to 0.001 mm or less. Furthermore, the squareness of the end surface of the bearing member located opposite to one of the both end surfaces of the flange portion via the thrust bearing clearance with respect to the inner circumferential surface of the bearing member can be set to 0.002 mm or less while the flatness thereof can be set to 0.0015 mm or less. Furthermore, the flatness of the inner surface of the bottom portion of the housing can be set to 0.002 mm or less.
Furthermore, the present invention provides a hydrodynamic bearing unit comprising: a housing with a bottom; a bearing member fixed to an inner circumference of the housing; a shaft member having a shaft portion inserted into the inner circumferential surface of the bearing member and a thrust plate provided on the shaft portion; a radial bearing portion provided between the inner circumferential surface of the bearing member and the outer circumferential surface of the shaft portion of the shaft member for supporting the shaft portion in a radial direction in non-contact state with a dynamic pressure action of fluid generated in a radial bearing clearance; and a thrust bearing portion respectively provided between each end surface of the thrust plate of the shaft member and the lower end surface of the bearing member or the bottom surface of the housing for supporting the thrust plate in a thrust direction in non-contact state with a dynamic pressure action of fluid generated in a thrust bearing clearance; wherein the surface hardness of the outer circumferential surface of the shaft portion is larger than that of the inner circumferential surface of the bearing member, the surface hardness of the both end surfaces of the thrust plate is larger than that of the lower end surface of the bearing member and the bottom surface of the housing, the surface roughness of the outer circumferential surface of the shaft portion is smaller than that of the inner circumferential surface of the bearing member, the surface roughness of the both end surfaces of the thrust plate is smaller than that of the lower end surface of the bearing member and the bottom surface of the housing, and the outer circumferential surface of the shaft portion has a surface characteristic on which fine projections constituting the surface roughness is smoothed. The surface with such surface characteristic can be formed through grinding process or the like followed by tumbler process, barrel process or the like. Alternatively the surface can be formed through grinding process or the like followed by relatively sliding process with the slide member with a surface hardness larger than the surface.
In the above structure, preferably, the outer circumferential surface of the shaft portion has a mean square inclination angle xcex94q, designated in ISO4287/1, of 2.0 or less.
In the above structure, the both end surfaces of the thrust plate can assume a structure in which the both end surfaces of the thrust plate has a surface characteristic on which fine projections constituting the roughness of the surface is smoothed. In this case, the both end surfaces of the thrust plate can have the mean square inclination angle xcex94q, designated in ISO4287/1, of 2.0 or less.
Furthermore, in the above structure, preferably the outer circumferential surface of the shaft portion has the arithmetic average deviation Ra, designated in ISO4287/1, of 0.04 xcexcm or less, while the both end surfaces of the thrust plate have the arithmetic average deviation Ra, ruled out in ISO4287/1, of 0.04 xcexcm or less, preferably Ra 0.01 xcexcm or less.